1. Field of Invention
This invention pertains, in general, to sensing far-infrared radiation, and in particular, to a photoconductor array for sensing radiation in the far infrared region of the electromagnetic spectrum.
2. Description of the Background Art
In general, sensing devices are designed to detect radiant energy found in certain regions of the electromagnetic spectrum. Development of a sensing device that detects all wavelengths of radiation from the electromagnetic spectrum can be quite daunting due to the size and complexity of such a device. Therefore, sensing device manufactures have produced devices that are sensitive to certain discrete regions of radiant energy, including far ultraviolet, near ultraviolet, visible, near infrared, and far infrared, within the electromagnetic spectrum. Manufacturing of sensing devices for each of these regions varies depending on the energy of light that is contained within each region. In some cases, like the detection of far infrared light, a sensor includes a one-dimensional line of light sensitive pixels or a two dimensional array of pixels comprised of stacked bars of light sensitive material. Once the incoming light has been detected, readout electronics process electric signals generated by the incident radiation interacting with the detecting material. The challenge of efficiently transferring information (in the form of electronic signals) from the detecting medium to the readout medium has confounded sensing device manufacturers for decades.
For conventional, far-infrared sensing devices, the interface between the detecting array and readout electronics is carried out by wire bonds connecting each detection pixel to the corresponding unit cell within the readout electronics. The wire bonds can be manufactured from an electrically conducting material like copper. However, the extreme size of the detecting and readout media, and, subsequently, the large number of pixels and unit cells to be connected, render wire bonding an inefficient means for interfacing the detecting array with the readout electronics.
Several other challenges also plague the far infrared sensing device community, including the inherent temperature increase of the sensing media, the inherent radiant glow from the readout multiplexer, and a possible thermal mismatch between the sensing material and the readout material. For near infrared sensing devices, an increase in detector temperature and a radiant glow from the readout medium is of little concern due to the high energy of the incoming radiation to be detected and the detectors operating temperature. Any incoming near infrared signal is significantly greater than the background signal generated by detector temperature and readout glow.
However, for far infrared detection, where incoming radiation is substantially less energetic than near infrared radiation and the detectors need to operate in much lower temperatures, readout glow and detector heating become problematic. Any stray light reaching the detecting surface due to glow from the readout multiplexer will compromise detector performance. In addition, the heat generated by the readout affects the performance of the detector array by raising its temperature.
This, in turn, causes an increase in excess dark current, thus resulting in the degradation of detector performance. Also, the effects of readout glow are especially pronounced in the low temperature environment necessary for far infrared detection.
In the case that the detecting media and readout media are constructed from different materials, a thermal mismatch is created. The detecting and readout materials contract and/or expand at different rates when exposed to the low temperatures required for far infrared sensing. When this happens, the overall yield of a detecting device can be significantly decreased.
Another difficulty that arises when detecting radiant energy of any particular wavelength relates to the presence of optical crosstalk between adjacent pixels on the surface of a detecting array. For a conventional detector array, typically a single wafer of some thickness is pixelized on one surface of the array by depositing metallic pads in a desired pattern. These metalized pads are the connections to the readout inputs. In the conventional design of linear arrays made of single bars of detector material, the pixelization is commonly accomplished by cutting grooves into the bar mainly in an effort to eliminate optical crosstalk. However, this approach to eliminating optical crosstalk is not conducive for implementation on large surface area arrays, like those needed for far infrared detection, due to the increase in processing costs for cutting grooves over such a large area.
Furthermore, another significant challenge facing far infrared sensor manufacturers involves maintaining a stable operating voltage for the detecting media during the time that the detecting media waits for its signal to be integrated by the readout media. When the overall voltage (or bias) of the detecting media becomes unstable, or debiased, a significant limitation in detector efficiency is encountered.
What is needed is a sensing device that will efficiently detect far infrared radiation that does not suffer from deficiencies introduced by a) detector heating b), readout glow, c) thermal mismatching, d) detector debiasing, and e) optical crosstalk.